Omega Engineering DAQ-12 user manual Programming the 8254 Counter/Timer

4.2Programming the 8254 Counter/Timer

This section provides programming information for the 8254 counter/timer as implemented on the DAQ-12. For more details on the 8254, consult the Intel Micro-processor and Peripheral Handbook.

To program any of the counters contained in the 8254 counter/timer, three steps are required:

1.Write the configuration byte to the 8254 mode select/status register. This byte sets the operating mode of the selected counter.

2.Write the least significant byte of the count value to the selected counter register.

3.Write the most significant byte of the count value to the selected counter register.

The following examples illustrate the programming sequence for each of the counters in the 8254. The variable 'base_address' is the base address of the DAQ-12 as defined by the address selection switches.

Counter 0 - Clock rate register (low word)

operating mode:		2
minimum count value:		2
configuration byte:		0 / 0 / 1 / 1 / 0 / 1 / 0 / 0 = 34H
Example:	Program the value 2675H into the low word of the clock rate register.
		output	34H	to	base_address + 0FH
		output	75H	to	base_address + 0CH
		output	26H	to	base_address + 0CH
Example:	Program the value 0008H into the low word of the clock rate register.
		output	34H	to	base_address + 0FH
		output	08H	to	base_address + 0CH
		output	00H	to	base_address + 0CH

DAQ-12 Users Manual 29